The present invention relates to methods and apparatus for the transmission of signals useful in seismic exploration both on land and in water covered areas, and also to methods and apparatus for deriving information from the cross correlation of the transmitted signals which are received after reflection from geological interfaces with replicas of the transmitted signals so as to delineate reflecting surfaces at different penetrations into the formation with high resolution.
In the search for high resolution seismograms, various types of signals have been designed for transmission into the ground in the form of acoustic energy. Classically these signals have been sinusoids which sweep in frequency linearly and monotonically with time (see, D. L. Goupillaud, Signal Design in the "Vibroseis" Technique, Geophysics, Vol. 41, No. 6, 1291-1304, December 1976. D. E. Nelson (U.S. Pat. No. 4,204,278, issued May 20, 1980), has described a signal design which can be generated and transmitted for geophysical exploration into the ground utilizing quasi-periodic pulse trains which are swept in repetition frequency over an octave monotonically and combined with the transmission through the medium (viz, after reflection from the geological reflecting surfaces) from which high resolution and accurate seismograms can be generated.
It has been found in accordance with this invention that a signal, with the energy content of Nelson's signal, and containing exactly the same pulses may be randomized (or pseudo randomized) such that its constituent parts (the individual durations or periods which vary in their time length, each constituting a signal making up the sweep or transmission) occur in differently scrambled order in each sweep and in successive sweeps so that no two sweeps in a set of sweeps are substantially correlated thereby to preclude false target indications and information that might erroneously locate or obscure geological reflecting surfaces. A signal provided in accordance with this invention when transmitted into the ground and received by receptors (for example in an array of geophones or hydrophones) possesses important advantages arising out of a lack of correlation between successive lengths (the pluralities of signals) which make up the sweeps or successive sweeps. These advantages include: (a) The ability to derive information from the received signals in which the shot point spacing and common depth point (CDP) multiplicity may be selected as desired after the survey is performed and may vary for different parts of the section; (b) The ability to sum the auto correlation functions (the cross correlation output of each sweep after transmission through the medium with its replica) and provide side lobes adjacent to the main lobe which are of the same order of amplitude as obtained with a monotonic sweep or less; and (c) The generation of side lobes in the correlation functions which are uncorrelated from sweep to sweep in successive seismograms, which with monotonic sweeps would be correlated, thereby further reducing side-lobe amplitude in gummed or stacked traces.
While various pseudo random sequences of sinusoidal waves or pulses have been proposed, neither the randomization of the signals in the transmission to preclude cross correlation both internally in a sweep and between successive sweeps nor the randomization of the signals in a transmission, as defined by Nelson, has even been suggested as being possible or practicable (see the article by P. L. Goupillaud referenced above and Crook et al, U.S. Pat. No. 3,264,606, issued Aug. 2, 1966; Forrester, U.S. Pat. No. 326,320 issued June 20, 1967; Barbier et al, U.S. Pat. No. 3,811,111 issued May 14, 1974; Barbier, U.S. Pat. No. 3,956,730, issued May 11, 1976; and Barbier, U.S. Pat. No. 4,011,924 issued Mar. 15, 1977).
In the randomization proposed by Goupillaud, because the successive pulses after randomization do not join smoothly at pulse boundaries, spurious frequencies are generated and the frequency range of the sweep differs from that of the monotonic sweeps. Thus the amplitude spectrum of two "randomized" sweeps intended to have the same range, will in fact be different. The orthogonality of randomized sweeps as suggested by Goupillaud is governed by pure chance and may in any given instance not be valid.
Accordingly, it is the principal object of the present invention to provide improved methods and apparatus for the transmission of signals suitable for seismic exploration and geophysical prospecting both on land and in water covered areas.
It is another object of the present invention to provide improved methods and apparatus for seismic prospecting in which the seismic signals are transmitted and processed so as to obtain seismograms having selected shot point spacing, sweep duration, and CDP multiplicity from the same transmission and without having to repeat the transmission.
It is a further object of the present invention to provide improved methods and apparatus for the transmission of seismic signals which enables the sweep duration to be selected either upon transmission or during processing after the survey is performed so as to obtain information in which the necessary compromise between the resolution, penetration and signal to noise ratio yields optimum results.
It is a still further object of this invention to provide a method of generating and applying a set of distinctly different, mutually orthogonal, seismic signals all having the same amplitude spectra.